I. Field of the Invention
The present invention relates to a valve operating mechanism for an internal combustion engine having a plurality of exhaust valves per cylinder.
II. Description of the Prior Art
In an internal combustion engine for automotive vehicles, etc., having a plurality of exhaust valves and a plurality of intake valves per cylinder, it has been proposed to operate at the different valve timings the exhaust valves and the intake valves, respectively.
A prior art valve operating mechanism of the above described kind, as for example disclosed in Japanese Patent Publication No. 49-17967, is adapted to operate two exhaust valves and two intake valves of a four-stroke cycle four-valves-per-cylinder internal combustion engine at the valve timings as shown in FIG. 1 of the Publication. That is, the valve timings E1 and E2 of the two exhaust valves and the valve timings I1 and I2 of the two intake valves respectively have such predetermined phase differences that at the positions of the piston adjacent the top dead center in the transition from the exhaust stroke to the intake stroke a set of one exhaust valve and one intake valve are closed and a set of the other exhaust valve and the other intake valve are opened with a view to increasing the velocity of flow of the fresh intake mixture within the cylinder and thereby improving the scavenging efficiency. Further, at the position of the piston adjacent the bottom dead center in the transition from the intake stroke to the compression stroke, only one of the intake valves is opened to cause a swirl of the intake mixture for thereby improving the combustibility during high-speed high-load operation of the engine.
In the valve operating mechanism of the above described type, a phase difference is provided to the timings at which the two exhaust valves fully close. Due to this, the effective area of the opening defined by the exhaust valves at the time of the valve overlap in which the intake and exhaust valves are held open simultaneously is small, thus resulting in that a long valve overlapping period is necessitated in order to fully attain an inertia supercharging effect of the intake mixture during high-speed high-load operation of the engine.
However, when the valve overlapping period is set to be long, the exhaust gases flow reversely into the cylinder, drawn by a large intake vacuum though the area of the opening defined by the exhaust valves, thus increasing the quantity of the residual gases during low-speed low-load operation of the engine in which the supply of intake mixture is small, thus largely deteriorating the combustibility.